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Title: Electricity in Locomotion
An Account of its Mechanism, its Achievements, and its Prospects
Author: Adam Gowens Whyte
Release Date: February 17, 2016 [EBook #51242]
Language: English
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The Cambridge Manuals of Science and Literature
ELECTRICITY IN LOCOMOTION
CAMBRIDGE UNIVERSITY PRESS
London: FETTER LANE, E.C.
C. F. CLAY, Manager
image
Edinburgh: 100, PRINCES STREET
London: H. K. LEWIS, 136, GOWER STREET, W.C.
Berlin: A. ASHER AND CO.
Leipzig: F. A. BROCKHAUS
New York: G. P. PUTNAM'S SONS
Bombay and Calcutta: MACMILLAN AND CO., Ltd.
All rights reserved
ELECTRICITY IN
LOCOMOTION
AN ACCOUNT OF
ITS MECHANISM,
ITS ACHIEVEMENTS, AND
ITS PROSPECTS
BY
ADAM GOWANS WHYTE, B.Sc.
Editor of Electrical Industries
and Electrics
Cambridge:
at the University Press
1911
TO
EMILE GARCKE
With the exception of the coat of arms at the foot, the design on the title page is a reproduction of one
used by the earliest known Cambridge printer, John Siberch, 1521.
PREFACE
In the following pages an attempt is made to give a clear picture of the part which electricity has taken and will continue
to take in the development of locomotion.
Some of the aspects of electric traction are highly technical; others are purely financial. It is impossible to understand the
achievements and possibilities of electricity in locomotion without a certain amount of discussion of both these points of
view; but it is not necessary to go deeply into either in order to catch some of the enthusiasm which inspires the
electrical engineer in his efforts to extend electric traction everywhere on road and rail. The hopes of electrical conquest
extend, indeed, to locomotion on the sea and in the air as well as on the land. At the root of these hopes there lies a firm
faith in the superior economies and flexibility of electricity as a mode of motion.
In the explanations which are given of electric tramways, electric railways, electric automobiles, electric propulsion on
ships, and the other phases of electric traction, nothing but the most elementary knowledge of electricity is presupposed.
A certain amount of technical description is unavoidable, but I have restricted it as far as possible to essential matters
which throw light upon the meaning of the various systems of electric traction and explain the economic and physical
reasons for their adoption.
Anyone who glances over the history of electric traction will be struck by the absence of outstanding names. There is no
man who occupies the same position in the sphere of electric locomotion as Watt does in the world of steam, or
Stephenson in the world of railways. As a pioneer, Dr. Wernher von Siemens perhaps deserves more honour than any
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other. But the leading ideas embodied in electric traction systems were contributed by engineers who worked in the
general field of electrical engineering; and they have been applied and developed by a numerous band of men who have
added one brick of experience and ingenuity to another until the imposing structure was made visible to the world.
Nevertheless, I hope the story as told briefly in the following chapters will not be found devoid of human interest. It has
the advantage, at any rate, of the attraction which anything pertaining to electricity holds for all sections of the public.
This attraction deepens upon closer acquaintance with the mechanism and the history of electricity in action; and if any
of the descriptions and forecasts are found to be prejudiced in favour of a single instrument of locomotion, the fault may
be considered to rest with the spell which electricity throws upon everyone who is concerned in any way with its
applications in the service of man.
I have to acknowledge the kind assistance of Mr. Frank Broadbent, M.I.E.E., in looking over the proofs of this volume.
A. G. W.
21 April 1911
CONTENTS
PAGE
Preface
v
CHAP.
I.
The Wheel and the Public
1
II.
Early Tramroads and Railways
4
III.
The Birth of Electric Traction
12
IV.
The Essential Advantages of Electric Traction on Tramways
19
V.
The Mechanism of an Electric Tramcar: the Overhead System
29
VI.
Conduit and Surface-Contact Tramway Systems
37
VII.
The Backwardness of Electric Traction in Great Britain
46
VIII.
Electric Tramway Stagnation. The Trolley Omnibus
55
IX.
Regenerative Control
67
X.
Accumulator Electric Traction. The Electric Automobile
70
XI.
Petrol-Electric Vehicles and main Marine Propulsion by Electricity
82
XII.
The Pioneer Electric Railways
92
XIII.
Electric Railways from the Engineering Point of View
107
XIV.
Electric Traction on Main Line Railways
116
XV.
Curiosities of Electric Traction
124
XVI.
The Future
138
Index
142
CHAPTER I
THE WHEEL AND THE PUBLIC
One of the greatest of unknown men of genius was the inventor of the wheel. Probably—as in the case of most
inventions—he shares the credit with others who prepared the way for him by discovering that heavy weights could be
more easily rolled than dragged. But, whatever the origin of the wheel and axle, the combination was so admirable that
it remained unchanged in its essential features for centuries and still forms the primary element in locomotion.
Some of the earliest forms of vehicle can be found co-existing with the very latest. In Oporto, for instance, there are
electric tramways, but there are also ox wagons which seem to belong to the childhood of the world. The wheels are
rigidly fixed to rotating axles (the oldest known arrangement) and the supports of both the front and the back axles are
rigidly fixed to the wagon. The result is that the vehicle cannot 'steer' and must be dragged round corners. Some time
ago the authorities, realising at last that this dragging was ruinous to the road surfaces, made a regulation that all wagons
should have their front axles pivoted. This attempt at improvement caused more agitation than the Revolution itself. The
owners of wagons argued—with perfect justice—that the rigid wagon had served for innumerable generations; and they
refused, in the face of fines, to make the change. Their resistance was so general and so dogged that the law became a
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dead letter, and the people reverted with great content to the ancient system which divided the business of local
transport between yoked oxen and women who had been trained from girlhood to carry heavy loads upon their heads.
This example of conservatism, though extreme, is characteristic of the attitude of the general public towards innovations
in locomotion. Until mechanical power came to be used, there was—for many centuries—nothing which could be
described as a radical innovation in transport. Roads were multiplied and improved; some advance was made in the
design and construction of carriages; and the organisation of posting and stage-coach services was developed. But little
more was done. Compared with these superficial changes, the idea of using steam power on the highway or on a
railroad was so drastic a change that it roused tremendous opposition. The railway companies fought this opposition
and overcame it, but the use of steam carriages on ordinary roads was postponed until the appearance of the petrol
motor encouraged a movement—once more against strong prejudice—for the repeal of the legislation which restricted
the use of mechanically-propelled vehicles on the roads. In a similar way horse tramways were violently attacked; and
their conversion to electric traction was opposed by a determined minority in every town. More recently, there was a
vigorous agitation against the substitution of motor omnibuses for horse omnibuses in London and elsewhere.
To some extent this recurrent opposition was reasonable enough. The new forms of locomotion had dangers of their
own; they were generally noisy and sometimes dirty; and occasionally, as in the case of early tramways, they were a
nuisance to existing traffic. But it may be noted that electricity claims to provide a means of locomotion not only more
rapid and more efficient (in most cases) than any other, but free from many of the drawbacks which gave conservatism
an excuse for opposing the introduction of steam and other forms of locomotion.
In the following pages I hope to give a clear account of the achievements of electricity in the field of locomotion and also
to indicate some of its more immediate potentialities.
CHAPTER II
EARLY TRAMROADS AND RAILWAYS
It has sometimes been remarked, by unfriendly critics, that tramways are an apology for bad roads. That is to say, if
road surfaces were perfect, there would be no need to lay rails in order to allow vehicles to run easily.
Although this view of the case may be no better than a quarter-truth, it is justified to the extent that tramways were, as a
matter of fact, the outcome of an attempt to escape from bad road surfaces. In the early days of mining, coals were
taken by horsedrawn wagons from the pits to the harbours. The passage and re-passage of heavy vehicles on the same
roadway led to the formation of deep ruts; and the first step towards both the tramway and the railway was taken when
logs of wood or 'trams' were laid in the ruts to facilitate transport.
The next step was to make the upper surface of the log round and the rims of the wheels hollow, so that they fitted over
the rails and kept the wagons on the track. Owing to the upper part of the rails wearing away quickly, thin plates of iron
were in some cases nailed to them. This improvement led to the adoption of a cast-iron rail, fastened to wooden
sleepers.
The earliest cast-iron railway was laid down before the middle of the eighteenth century, about one hundred years after
the first wooden 'tram-ways.' Half a century later we find the first rail-and-wheel combination as we know it on modern
tramways and railways, where the wheel carries an inner flange and runs upon the head of a narrow metal rail. This is
the form which experience has proved to be best adapted for safety, speed, and economy in power. The improvements
made since the beginning of the nineteenth century have been in matters of detail.
Many miles of colliery tramroads were in existence when—at the beginning of the nineteenth century—the idea of using
the steam engine in place of the horse was taken up by engineers. They were concerned at first solely with the carriage
of coal; the idea of conveying passengers arose at a later date, after the steam automobile had been tried and
abandoned for the time being. George Stephenson, for instance, ran his first locomotives on colliery tramroads; and the
first railway—between Stockton and Darlington—was used for passengers merely as an afterthought. It was, in fact,
designed to be a tramroad for the use of the public in general transport by horse traction.
The most curious feature of this stage in the evolution of locomotion was that, although Stephenson's locomotives had
been at work for several years and although several schemes of iron roads had been projected, very few people had
any conception of the development awaiting the locomotive and iron road in combination. They did not even appreciate
the proved fact that the locomotive was a more efficient means of transport than the horse. An immense amount of
pioneering work had to be done before the impression of a new era could be borne in upon the public mind. These
were the days when the Quarterly Review backed 'old Father Thames against the Woolwich Railway for any sum' and
when a witness before a Parliamentary Committee (on the Liverpool and Manchester Railway Bill, in 1825) thought
himself safe in suggesting that a steam locomotive could not start against a gale of wind.
When these prejudices were overcome, many years had to pass before the objections of landowners and citizens were
worn down. Railway engineers spent most of their time in a form of diplomatic warfare with opponents to their
schemes; huge sums—part of which still lingers in the capital accounts of railway companies—were spent in
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Parliamentary proceedings over Railway Bills. This barren process had to be repeated when electric traction made its
appearance; but happily the electrical fight was not upon quite so extensive a scale, nor was the period of preparation
followed by anything comparable to the Railway Mania of 1845, when the public made up for its early contempt of
railway enterprise by tumbling over itself to get shares in some of the most crazy schemes which were ever put into
shape by unscrupulous company promoters.
The early history of the steam railway is interesting in connection with electrical locomotion for two reasons. It shows
that the railroad proper evolved out of the tramroad or 'light railway,' as it would now be called—a type of line which is
specially suited to electrical operation. It also includes a controversy between three modes of traction; and this
controversy forms a very good introduction to a discussion of the reasons why electricity is so economical in
locomotion.
These three modes were (1) stationary engines: (2) locomotives: (3) the device known as the 'atmospheric railway.'
In both the first and third, engine houses were placed close to the line at convenient intervals. In the first, each steam
engine operated an endless rope to which the train of carriages was attached. The system is still in use for colliery
working and is also employed (in an improved form, of course) for funicular railways. George Stephenson himself
employed it to assist locomotives up heavy gradients. In the atmospheric railway the stationary engines were used to
exhaust the air from a length of cast-iron piping laid close to the railway. The principle is the same as that of the
'pneumatic tube' which the Post Office uses for sending papers over short distances. The papers are placed in a cylinder
which fits the interior of the tube; and when the air is exhausted from the tube in front of the cylinder, the pressure of the
air behind it drives the cylinder forward.
Nowadays it is difficult to realise that such a system was seriously proposed for railway work and actually adopted by
an engineer of such eminence as Brunel. But in point of fact it was recommended by two Board of Trade experts in
1842 and by a Select Committee appointed in 1845 to consider several Bills for atmospheric railways. It was tried at
Dalkey and Croydon, and it was installed under Brunel's supervision on a six-mile line in Devon. The carrier in the tube
was connected to the train through a longitudinal slit at the top of the tube. The slit was closed by a leather flap, except
when momentarily lifted by the passage of the train. A great deal of ingenuity was exhausted in attempting to make this
'longitudinal valve' efficient, but it was found that heat, moisture, and frost made the leather deteriorate so rapidly as to
render it hopelessly ineffective in a short time. After a series of misfortunes the atmospheric railway became a mere
curiosity in the history of invention.
Stephenson was right in regarding the atmospheric railway as 'only the fixed engine and ropes over again, in another
form.' He was also right in his belief that the steam locomotive was more economical than either of its rivals. But the
stationary engine idea had the germ of an even sounder principle than that of the locomotive. Both in electric tramways
and electric railways the power is obtained from stationary engines. The main difference between the electric system
and the old rope and atmospheric systems lies in the superior economy with which the power is conveyed electrically to
the trains. There are other important differences; but the essential point is that both rope traction and pneumatic
propulsion wasted so much power between the engine and the train that their other advantages were annulled, and it
was found cheaper to put the engine on wheels and make it drag itself as well as the train.
Brunel's reasons for his faith in the atmospheric railway are well worth quoting for the light they throw indirectly upon
the advantages of electric traction. He argued that stationary power, if freed from incumbrances such as the friction and
dead weight of a rope, was superior to locomotive power, on the following grounds:
(a) A given amount of power may be supplied by a stationary engine at a less cost than if supplied by a locomotive.
(b) The dead weight of a locomotive forms a large proportion of the whole travelling load, and thus inherently involves a
proportionate waste of power—a waste which is enhanced by the steepness of the gradients and the speed of the
trains.
Experience has proved the soundness of these principles. There has been a steady improvement in the power and
efficiency of locomotives, but progress has reached a point at which further increases in speed and accelerating power
(a very important matter) are not attainable without a prohibitive increase in the consumption of coal and a costly
strengthening of the railway track to stand the strain of heavier engines pounding along at very high speeds. Electric
traction, which is a reversion in part to the stationary engine system, offers a means of escape from the limitations of the
locomotive.
There is still some doubt in the minds of railway engineers whether electric traction is really superior to the steam
locomotive on the main railway lines, where distances are great and train loads heavy. But the superiority is admitted on
suburban lines and also on tramways, where electricity has almost completely supplanted both horse and steam traction.
If Brunel had foreseen how economical electricity would be in the transmission of power between engine and train, he
would have felt still more confident in his defence of the stationary engine.
CHAPTER III
THE BIRTH OF ELECTRIC TRACTION
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The story of electric traction really begins in the laboratory of Faraday. He was the first to produce mechanical rotation
by electrical means; and, although he had no practical end in view, his investigations produced the germ of the
commercial dynamo and thence of the commercial electric motor.
That germ, however, took about half a century to develop. It is true that in 1837 (about ten years after Faraday's
discovery) Robert Davidson experimented with an electric locomotive on the Edinburgh and Glasgow Railway; it is also
true that Jacobi, two years later, propelled a boat on the Neva with electric power. But these early attempts were not
on a commercial scale. Not only was the motor a crude contrivance, but the method of producing the electric power
was hopelessly extravagant.
At that period the 'primary battery'—similar in character to those still used for laboratory purposes, ringing electric bells,
and so on—was the best available source of electricity. Such batteries generate current by the chemical consumption of
zinc. In order to obtain sufficient power to move a boat, a large number of batteries had to be coupled together. They
were expensive in first cost, expensive in the zinc which was their 'fuel'; and they became rapidly exhausted.
Dynamo
Fig. 1. Diagram to illustrate the essential identity of the dynamo and the motor. The dynamo generates
electricity when the armature or group of coils is forcibly revolved close to magnets, thus converting
mechanical energy into electrical energy. The motor causes its armature to revolve forcibly when
current is supplied to it from the dynamo. Thus the motor converts electrical energy into mechanical
energy.
The essential step towards the commercial plane was taken when an efficient means was devised for transforming
mechanical into electrical energy on a large scale. The first 'dynamo-electric' machines, invented about the middle of last
century, were merely hand machines. Their power was limited by the strength of the permanent magnets employed in
their construction; and although an increase in power was obtained by multiplying the number of magnets and driving by
steam power, it was not sufficient for commercial purposes. In 1867 electro-magnets were first employed by Siemens
and Wheatstone; and from this application there was developed a machine whose power as a generator of electricity
was limited only by its size and the speed at which it was run.
It is unnecessary for our present purpose to enter into the technical details of the modern electric generator and the
modern electric motor. The principles underlying them are quite simple, although the theory of their design and the
practice of their construction and operation are almost a science in themselves. A dynamo or electric generator is a
machine for transforming mechanical into electrical energy; an electric motor is a machine for transforming electrical
energy into mechanical energy. If, therefore, we place an electric motor upon a vehicle and supply it continuously with
current from a dynamo, the motor will rotate and can be used to propel the vehicle. That is the essential mechanism of
electric traction.
The simplicity of the arrangement is enhanced by the fact that the dynamo and the motor are virtually the same machine.
In the dynamo, a cylindrical 'armature' of coils is forced to rotate close to the poles of electro-magnets; the energy
exerted in turning the armature against the influence of the electro-magnets is transformed into the energy of electric
currents in the coils of the armature. In the motor, which also consists of an armature close to the poles of electro-
magnets, the process is reversed. When a current is passed through the coils of the armature, the reaction between
these currents and the electro-magnets causes the armature to revolve.
This reversibility of the dynamo was, according to a story frequently repeated, first discovered quite by accident. In a
Paris exhibition a number of Gramme dynamos—or dynamo-electric machines, as they were then called—were being
separately connected to lamps and other devices for showing the effect of electric currents; and when one was started
up it was found that another was being driven at a rapid rate. Investigation showed that the second one had been
coupled up to the first by mistake and was therefore being worked as a motor by it.
This was in the year 1879; and the story of the incident served to draw general attention to the discovery of a new and
efficient means of transmitting power. Engineers recognised that in the steam-driven dynamo they had the means of
producing powerful electric currents, while in the electric motor, connected by wires to the dynamo, they had the means
of reproducing the power in mechanical form at a distance. There were, of course, losses of energy in the process. A
certain percentage was lost in the dynamo itself, some in the transmitting wires, and some in the motor. But the all-round
efficiency of the arrangement was much higher than that of any other system of transmitting power from one point to
another several miles distant.
In order to apply this system to propelling vehicles it was only necessary to devise a continuous connection between the
motor on the vehicle and the stationary dynamo. This was done on the first electric railway by means of a 'third rail,'
substantially in the same way as is now familiar on underground and other electric lines. The third rail was a metal
conductor supported on insulators and connected to the dynamo. The vehicle or car was furnished with a metal brush
or skate which rubbed along the third rail as the car moved forward. The current thus collected was led through the
motor (which drove the axle of the car through toothed wheels) and thence to the track rails, which conveyed the
current back to the dynamo and so completed the electrical circuit. Messrs Siemens and Halske exhibited the first
electric railway of this type at the Berlin Industrial Exhibition of 1879.
Another method of collecting the current was tried soon afterwards and formed the direct forerunner of the electric
tramway on the now standard 'overhead' system. The disadvantage of the third rail system is that it involves an exposed
'live' conductor close to the ground. It is therefore quite unsuited for use on streets. Consequently the next step towards
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the electric tramway was to carry the electrical conductors overhead by supporting them on poles erected at the side of
the track. The first installation of this kind was laid down at the Paris Exhibition of 1881. In that case the conductor was
an iron tube with a slot along its lower side; and inside the tube was a 'boat' which slid along and was connected to the
car by means of a flexible wire. A second tube, also with a boat and connecting wire, was provided to carry the return
current. We shall see later how this arrangement evolved into the familiar 'trolley' system.
The mention of a slotted tube recalls the atmospheric system and, in so doing, emphasises the superiority of the electric
system in simplicity, flexibility, reliability, and economy. Brunel's faith in the advantages of stationary engines and the
transmission of power therefrom to moving trains would have been justified by the event if the pneumatic system of
power transmission had been as practicable as the electric system. But there is an obvious contrast between the huge
pipe of the atmospheric railway, with its impossible 'longitudinal valve,' and the small tube of the first overhead electric
line or the third rail of the first electric railway. There is also a pathetic contrast between the prolonged struggles which
Brunel and the inventors of the atmospheric system underwent before they were forced to acknowledge failure, and the
rapid ease with which electric traction entered into its kingdom when the commercial dynamo and motor were first
produced. The intrinsic difficulties which electric traction engineers had to meet were not serious. Designers passed,
step by step, from the model electric railway at the Berlin Exhibition to public lines on a larger scale, and from the model
electric overhead tramway to the 'street railway' or tramway which gradually supplanted the horse tramway. Each step
consisted in an extension of the distance covered and an increase in the power required, coincident with a gradual
improvement in the details of motors, dynamos, and transmission equipment.
CHAPTER IV
THE ESSENTIAL ADVANTAGES OF ELECTRIC TRACTION ON TRAMWAYS
A railway journal once committed itself to the statement that horse traction was superior to electric traction on roads
because the horse possessed the 'vital principle' of energy in its constitution.
It is distinctly curious to find an authority on locomotion describing the essential drawback of horse traction as its
distinguishing advantage. The 'vital principle,' unfortunately, needs food and rest to maintain it not only during working
hours but during the hours of inactivity as well. In actual practice four horses out of every five in a tramway stud are in
the stables while the fifth is at work. Moreover, the same stud has to be kept up, at a practically uniform cost, whether
the daily traffic be light or heavy. Thirdly, the 'vital principle' has only a limited number of years during which—apart
from sickness and disease—it is effective for traction purposes.
Tramway
Fig. 2. A typical electric tramway on the overhead system.—The trolley standard carries the wires for
supplying current to the cars on both the up and down tracks. The driver has his left hand on the
controller handle and his right hand on the brake handle. (Photograph reproduced by courtesy of Dick,
Kerr and Company, Limited.)
Another disadvantage is that the pull which a horse can actually exercise on a car is strictly limited and is only a small
fraction of the total power represented by the fodder which the horse consumes. The strain upon a horse in starting a
car or omnibus is so great that a 'lover of animals' used to supply London omnibuses with appeals to passengers not to
stop the omnibus more often than was necessary, especially on an incline. This was a recognition of the fact that the
horse cannot cope easily with the heavy strain at starting, and that he requires assistance on heavy gradients.
It was not surprising, therefore, that on horse tramway systems the speed was low, the cars of limited capacity, and the
fares comparatively high. The shortness of the journey which a tramway horse was able to cover without fatigue also
tended to limit the length of routes.
On all these points electric traction was soon found to be distinctly superior to horse traction. It was more economical in
power; it was able to maintain higher speeds with larger and more commodious cars; and there was no narrow limit to
the length of routes or the gradients which could be surmounted. Consequently electric traction offered the public an
improved service at lower fares.
The whole of the power-producing plant for a typical electric tramway system is concentrated at a generating station
placed (if possible) near the centre of the system. From this station runs a network of electric mains to feed the lines
with current at convenient points. This concentration is a benefit on several grounds. A large generating equipment is
cheaper in first cost than a multitude of small power-producing plants, and it is much more economical in operation. If
every car had its own power equipment, that equipment would need to be powerful enough to haul itself and the loaded
car up the steepest gradient on the route. That is to say, the sum of the car capacities would be equal to the sum of the
maximum demands. But when the power is obtained from a single stationary source we do away with the dead weight
of the power equipment on the car, and secure the very vital advantage that the capacity of the stationary source need
not be so great as the sum of the maximum demands. In actual working it never happens that all the cars are full of
passengers and ascending the steepest gradients simultaneously. While some are running up-hill, others are going down-
hill; while some are full, others are half full or almost empty. The result is that the total demand for power at any time is
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always very much less than the total of the maximum demands made by each car; and the capacity of the generating
station need be sufficient to cope only with the smaller amount.
This advantage reduces the expenditure necessary upon boilers, engines, and dynamos at the tramway generating
station. And it is enhanced by two valuable capabilities of the electric motor. The first is its power of taking a heavy
overload for a limited period without injury. There is no difficulty about making an electric motor, whose normal
capacity is 20 horse power, give 40 horse power momentarily, 30 horse power for several minutes, and 25 horse
power during the best part of an hour. Applied to tramway work, this advantage means that the rated capacity of the
motor equipment of a car may be less than what is required to haul a loaded car at an adequate speed up the steepest
gradient on the system. Such maximum demands, which only occur at intervals with each car, can be met by the
readiness of the electric motor for overwork. The motors may therefore be reduced in size, saving money in first cost
and in the current consumed.
The second valuable peculiarity of the electric motor is that it gives its 'maximum torque' at starting. That is to say, it
exercises the highest propulsive effort at the precise moment when it is required. When horses are employed, they have
to endure an abnormal strain in overcoming the inertia of a stationary vehicle; everyone must have noticed how horses
have to struggle to start a car which they can keep going at an easy trot once it has got up speed. The electric motor—
to use an apparent paradox—gives this abnormal pull as part of its normal action. As the inertia of the car is gradually
overcome, the speed of rotation of the motor increases and its torque decreases, automatically and precisely in
accordance with the demands of the case.
The starting torque of a motor is such an emphatic phenomenon that the driver of an electric car may, if he is careless
and switches the current on too suddenly, jerk any standing passenger off his feet, even though the total weight of the
car may be ten tons or more. Properly employed, however, the electric motor gives an even and rapid acceleration.
This is a far more important point in tramway economics than it appears to be at first sight. The superiority of the
electric tramway over the horse tramway depends less upon higher speed than upon the fact that less time is wasted in
stopping to pick up and set down passengers. Time is the vital element in all transport, and it is especially vital in
connection with tramways, which have to stop with great frequency. If the time which elapses between putting on the
brakes at each stop and getting up to full speed again can be materially shortened, then the average speed of the
tramway journey can be materially raised. It is easy, by means of powerful brakes, to bring a car to rest quickly; the
electric motor enables speed to be regained quickly. In this way a high average speed may be maintained in spite of
numerous stops; and, with larger cars, the electric tramway is able to handle a larger volume of traffic in a shorter space
of time than the horse tramway.
The time lost in stopping is of so much consequence that, when electric tramways were introduced, the old custom of
stopping the cars at any desired point was abandoned. Stopping places were arranged at convenient points along the
route, some of them being regular stops and others optional at a signal from passengers desiring to alight or to board the
car. The public soon got used to walking a short distance to a stopping place, although they did not, perhaps,
appreciate how much the collection of traffic at a reduced number of points tended to improve the general tramway
service.
A high average speed with numerous stops was, however, only one of the improvements which the public derived from
electric traction. Tramway passengers expect to find a car not only at a convenient point but within a convenient period
of waiting. With electric traction the service became much more frequent than with horse traction. It is quite possible to
run a horse tramway service profitably with cars at intervals of fifteen to thirty minutes, if the passengers are patient
enough to wait and fill each vehicle. But with electric traction the main item is the cost of the standing equipment—the
power house, mains, and overhead lines—and unless that equipment is adequately utilised the revenue will not cover the
standing charges. A fifteen-minute service is, generally speaking, the lowest economic limit on an electric tramway.
Every tramway manager tries to attract sufficient passengers for a more frequent service; and, as a matter of fact, it was
found that where there was sufficient population the provision of a frequent and rapid service encouraged tramway
travelling so much that cars had to be run at far shorter intervals than had been customary on horse tramways.
The increase of traffic brought with it the demand for larger as well as speedier cars with a shorter 'headway' or interval
between one car and another. The capacity of a horse car is limited by the fact that it is not convenient to harness more
than two horses to a single vehicle. But with electric cars there is no extraneous limitation to carrying capacity. Large
double-decked cars with seats for seventy passengers are now quite common. In America it is a frequent practice to
attach 'trailers' to the cars, making a short tramway train. Experiments have recently been arranged on similar lines in
London, for the handling of the heavy traffic at rush hours. These instances show that electric tramway capacity is
flexible and may be adjusted to the density and the fluctuating character of the demand.
Finally, it falls to be noted that the power consumed by a tramcar is, roughly, proportional to the useful work which the
car performs. As already mentioned, it costs about as much to work a horse tramway when the cars are empty as when
they are full, since the main item is the maintenance of the 'vital principle' of a certain number of horses independently of
the traffic. But with electric traction the motors require less power when the cars are running light. And less current for
the motors means less current generated at the power station—that is to say, less steam, less oil, less coal, less wear
and tear. If more current is demanded, it is because more passengers are being carried and more revenue earned.
Reviewing the subject broadly, it is apparent that the adoption of electric traction on a tramway is not so much a step in
advance as a beneficent revolution. The higher speeds with more frequent, more comfortable, and more commodious
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cars have created a volume of traffic far beyond what could have been handled with horse traction. The change also led
to a great increase in the length of tramway routes and to the construction of new tramway systems. In 1898, when the
electric tramway movement began in earnest, there were 1064 miles of tramway in the United Kingdom. Now there are
2562 miles, and the number of tramway passengers is more than double the total of third class passengers on the whole
system of British railways. The number of tramway passengers carried during 1909-10 (the last period covered by the
published official returns) was equal to about 62 times the estimated population of the United Kingdom.
While the traffic has multiplied in this remarkable fashion, there has been a heavy reduction in the fares charged. This
has been made possible by the economical features of electric traction. In the old days a horse tramway had to spend
about £80 to earn £100; an electric tramway need spend only about £60. With this reduction in the proportion of
expenses to receipts, and with the greater volume of business, it became feasible to stimulate traffic still further by giving
passengers much longer distances for their money. In fact, electric traction proved so economical that people began to
imagine that there was no limit to the reductions which might be made with financial safety. However, there is plenty of
evidence that a limit exists. In many cases it has been touched, if not passed, but the public continues to clamour for all
sorts of concessions. These demands are a great compliment to electric traction, but they are a decided embarrassment
to the tramway manager who believes in a reasonable margin between his total expenses and his total revenue.
CHAPTER V
THE MECHANISM OF AN ELECTRIC TRAMCAR: THE OVERHEAD SYSTEM
A rough idea has already been given of the elementary mechanism of electric traction—the combination of generating
station, of cars fitted with electric motors, and of a sliding contact between the two. It is in connection with the sliding
contact that the ingenuity of tramway engineers has been mainly exercised. Three distinct solutions were evolved for
tramway work, giving rise to three systems—(1) the overhead or trolley system; (2) the conduit system; and (3) the
surface-contact system.
The first system is now almost universal in the United Kingdom. Part of the London system is equipped on the conduit
system; and the tramways at Lincoln and Wolverhampton are constructed on the surface-contact system. Beyond these
cases the trolley holds the field. In the United States and on the Continent there is a larger proportion of conduit work,
but from a practical point of view it would hardly be necessary to mention either conduit or surface-contact if it were
not for the great engineering interest which they possess and for the controversies to which they have given rise.
Overhead system
Fig. 3. Diagrammatic illustration of the general arrangement of an electric tramway on the overhead system.
At the foot is shown the generating station which supplies alternating current at high-pressure (for
economy in transmission) to a sub-station where it is 'transformed' to low pressure and 'converted' in a
motor-generator to continuous current for distribution to the trolley wire from which each car takes its
current. The course of the current through the trolley pole and controller and thence to the motors and
back by the rails is indicated by arrows.
The overhead system has conquered because it is cheapest in first cost, cheapest to maintain, most economical in
current, and most reliable in action. Later developments in surface-contact traction have run it very close on some of
these points, but have not—for reasons which will be explained—affected the established position of the overhead
system.
In its essential features the overhead system has not altered very much from the experimental line erected at the Paris
Exhibition of 1881. The slotted tube has been replaced by a solid copper wire; and the 'boat' sliding within it has been
replaced by a wheel or a bow pressed against the lower side of the wire by means of a pivoted arm controlled by
springs. The sliding bow is common on the Continent, but it has been adopted on only one British tramway—that at
Sheerness. Its use for electric traction on railways will be mentioned later, but as far as British tramways are concerned
the bow is the exception which proves the trolley wheel rule.
The function of the trolley wheel is to collect current from the wire along which it rolls. This current passes through
insulated wires down the trolley arm to the controller, which the driver of the car operates by means of a handle. The
controller, which is really a series of electrical resistances, is analogous to a water tap. By its means the current may be
completely shut off from the motors, or allowed to flow in varying degree as required by the speed of the car. In starting
a car, the driver moves the controller handle notch by notch, so as to get a uniform rise in speed until the full current is
allowed to pass through the motors. With such a mechanism, supplemented by brakes, the driver has the movements of
the car under control.
In a four-wheeled car, each axle is driven by a motor. In a bogie car (one with a set of four wheels at each end) the
axles of the larger wheels of the bogie are each driven by a motor; but not directly. Considerations of space make it
necessary to keep the motor as small as possible, but if a motor is to be small and also powerful it must rotate at a high
speed. On the tramcar, therefore, the motor drives a small toothed wheel which drives a large toothed wheel fixed to
the axle, thus effecting a reduction of speed between the motor and the wheel.
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The same considerations of space join with others in making two motors on each car the general rule. And the use of
two motors enabled the tramway engineer to introduce a refinement into the method of control. This refinement is
known as the 'series-parallel system.' One of its objects is to give a large 'starting torque' and so enable the car to gain
speed quickly. When the current is first switched on by the controller it passes through the motors in tandem or in
'series,' thus dividing the pressure of the current (analogous to a 'head' of water) between them. The starting torque of a
tramway motor (or the turning moment which it exerts when current is first passed through it) is dependent on the
current but independent of the pressure. Thus the tandem or 'series' arrangement, which passes the full current through
each motor, gives the maximum starting torque without an undue consumption of current. After the car is well started,
the next movement of the controller puts the motors in 'parallel,' opening up two paths for the current instead of one, so
that each motor receives the full pressure. The practical result is that there is a very rapid acceleration at starting, with
marked economy in current. If the motors were kept in 'parallel' right through, twice as much current would be required
to get the same starting torque. It will be seen later how valuable this arrangement for getting a rapid start, without
excessive current consumption, may be in improving the physical and economic conditions of a tramway or train
service.
After having passed through the motors and done its work, the current is led to the wheels of the car and returns by
way of the rails, which are linked together by copper bonds so as to form a continuous conductor. The passage of the
current from the wheel to the rail is indicated by sparks when the rails are rough or very dry and dirty. Although the
rails, like the overhead wires, are thus carrying current, there is no danger of shock from them, as the electrical pressure
in them is only a few volts, at the outside, while the pressure in the overhead wires is 500 volts. It is this difference of
pressure which—like the 'head' of water in a turbine—supplies the motive power for the car.
Each car on a tramway system may thus be regarded as a bridge which completes an electrical circuit. When the driver
moves his controller, current flows from the generating station at a high pressure, passes through the controller, operates
the motors, and returns to the generating station at a low pressure. This typical circuit is completed through every car,
so that the demand on the generating station at any moment is the sum of the demands of the cars at that moment. The
business of the engineer at the generating station is to maintain the electrical pressure in the overhead wire at the normal
level of 500 volts; and in order to do this on an ordinary tramway system it is found convenient to divide the overhead
wire into half-mile sections, each of which has a separate main or 'feeder' from the generating station. The passenger
can detect the change from one section to another by the click of the trolley wheel across the gap which insulates one
half-mile section from another. At the same spot he can see the short square 'feeder-pillar' at the roadside (containing
the switches by which current can be turned off from that section) and the cables which pass along the arm of the trolley
standard and terminate in the overhead wire.
On an extensive tramway system the power-supply arrangements become more complicated. The central generating
station remains the primary source of power, but sub-stations are erected at convenient points between the central
station and the outskirts of the tramway area. These sub-stations are secondary stations for the distribution of electricity.
They receive power at extra-high pressure (5000 volts or more) from the central station; they contain special machinery
for reducing the pressure to 500 volts for distribution to the various tramway feeders. The object of this arrangement is
partly technical but mainly economical. Electric power can be transmitted at a lower cost in mains and with less loss of
energy at high pressures than at low. Consequently when the termini of tramway routes are several miles from the
generating centre, greater all-round efficiency is secured by transmitting current at high pressure to a number of well
selected sub-stations.
Pantograph
Fig. 4. Photograph of a car on a conduit section of the London County Council tramways. The centre line on
the vacant track indicates the slot rail through which the 'plough' on the car passes to make contact
with the conductors in the underground conduit. (Photograph reproduced by courtesy of Dick, Kerr
and Company, Ltd.)
CHAPTER VI
CONDUIT AND SURFACE-CONTACT TRAMWAY SYSTEMS
Roughly speaking, the arrangements for generating electricity, distributing it, and utilising it on the car, remain the same in
conduit tramways and surface-contact tramways as on the overhead system. The differences between the three systems
are, as already indicated, confined to the means of collecting the current for each car.
Both the conduit and the surface-contact system were suggested as a means of escape from the main objection to the
overhead system—the exposure of 'live' wires in the street. The cable tramway, with its concrete trough and slot, gave
an obvious hint. There would be no difficulty, apparently, in carrying wires on insulators in the trough or conduit, and
utilising the slot for a 'plough' which would slide along inside the conduit, keeping contact with the wires, and so
conveying the current to the car.
This was tried for the first time in Blackpool, where—in 1884—a length of conduit tramway was laid along the front
street of the town. The conditions could hardly have been less favourable for the system, as the sea frequently washed
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over the roadway, flooding the conduit with water and sand. Further, the conduit was so shallow that children were able
to get at the conductors with their metal spades. As the conduit carried the return wire, the effect of a metallic contact
between the two conductors was to cause a 'short circuit,' with very entertaining fireworks but with no amusing results
for the tramway engineer....
